Bacillus thuringiensis - sold for box trees and hedges, also kills brassica caterpillars.

If the link to ProGreen is broken, search for 'Bacillus thuringiensis caterpillars' or 'Dipel'.

Instructions for use: 

Mix one level teaspoon to half a litre of water, and stir well. Then add another half litre of water and apply straight away. Spray over the tops of brassica plants, no need to spray under leaves. Bacillus thuringiensis makes the leaves indigestible to caterpillars only.